Depressurising system for plants operating with pressurized steam

ABSTRACT

A depressurizing system (1) for depressurizing plants, operating with pressurized steam and including a steam head (16), by injection of cold water under gravity from a reservoir (2) located at a higher level than the pressurized plant and connected thereto by a delivery duct (24) which forms a syphon (24a, 24b, 24c) and which is connected to the narrow section (10) of an ejector (6) into which steam flows in the event of an incident, the steam being drawn from a condenser (7) located downstream of the ejector (6), causing a drop in pressure which draws cold water from the tank (2) through the syphon which is thus overcome, enabling cold water to be injected into the pressurized plant.

The present invention relates to a system for depressurising plantsoperating with pressurised steam.

The system is of the type in which cold water, contained in a reservoirat a higher level than the pressurised plant, is injected under gravity.

It is known that this type of system is used to ensure the maximumsafety in the case of an incident in a pressurised plant which requiresthe immediate depressurising of the plant itself.

A typical application is in the field of water-cooled nuclear reactorswhere research into more advanced safety measures is a fundamental partof any development programmed.

The depressurisation consists of the injection into the pressurisedplant of a large quantity of cold water which condenses the steampresent and lowers the temperature with a consequent drop in theinternal pressure.

After this depressurisation it is then possible to activate furthersystems for alleviating the incident.

The depressurising system must, however, activate without anypossibility of failure.

Such failure could have devastating effects in that the furtheralleviating systems would be ineffective.

Depressurising systems which operate by injection of a coolant liquidunder gravity are known.

Such injection is, however, initiated through the use of automatic logicenabling signals from a control systems or manually by operators. Thisthen requires the intervention of suitable actuator members such asvalves.

The need for an enabling signal, itself, however, introduces aprobability of failure.

Moreover the activation of the injection requires the presence ofexternal energy sources. This also introduces a failure probability.

Finally the actuators also have a failure probability.

For these reasons, recent design developments in the nuclear field havebeen directed to the devising of passive safety systems.

Such systems are able to carry out the functions assigned to them onlyon the basis of physical laws by virtue of adequatemechanical-functional design.

The technical problem at the basis of the present invention is that ofproviding a depressurising system for pressurised plants which overcomesthe above problems explained in the prior art, while satisfying thisrequirement.

This problem is solved according to the invention by a depressurisingsystem for plants operating with pressurised steam and including a steamhead, by injection of cold water under gravity from a reservoir locatedat a position higher than the pressurised plant and having a deliveryduct for delivery to the pressurised plant, characterised in that itincludes an ejector which has an inlet section, an outlet section and anarrow section, a condenser which has an inlet and an outlet, a firstconnector duct which puts the reservoir in communication with the steamhead, a second connector duct which puts the inlet section of theejector in communication with the steam head, a third connector ductwhich puts the outlet section in communication with the inlet of thecondenser, a first injector duct which puts the outlet of the condenserin communication with the pressurised plant, and in that the deliveryduct forms a syphon, the reservoir communicating with the pressurisedplant through the delivery duct, and through the narrow section of theejector.

The main advantage of the present invention lies in the fact that itoperates passively, relying solely on physical laws by virtue solely ofits structural characteristics.

A further advantage is due to the fact that the realisation of theinvention requires the assembly of components which are simple todesign.

A further advantage of the present invention lies in the fact that it isadapted to a wide range of pressurised plant including any type ofpressurised water or boiling water nuclear reactor.

Further characteristics and advantages of the invention will becomeclearer from the description of one embodiment of the depressurisingsystem for pressurised plants applied to a pressurised light-waternuclear reactor made below with reference to the appended drawings,given by way of non-limitative example.

In these drawings:

FIG. 1 shows schematically an embodiment of the depressurising systemfor pressurised plants, in accordance with the invention;

FIG. 2 is a schematic view of a different embodiment of thedepressurising system of the invention;

FIG. 3 is a schematic view of a further version of the depressurisingsystem of the invention.

In the drawings, a depressurising system for pressurised plants isgenerally indicated 1.

The pressurised plant is constituted, in this embodiment, by a nuclearreactor which is not shown in its entirety although components whichinteract with the depressurising system 1 are shown.

The system 1 includes a tank 2 filled to a level 3 with cold waterwhich, in a preferred embodiment of the invention, contains an elementfor inhibiting the nuclear fission reaction, such as boron, in solution.

The level 3 divides the tank 2 into an upper part 4 and a lower part 5.

The depressurising system 1 also includes an ejector 6 and a condenser7.

The ejector 6 has an inlet section 8, an outlet section 9 and a narrowsection 10.

The condenser 7, preferably of the straight-tube type 30, has an inlet11 and an outlet 12.

The nuclear reactor includes a pressuriser 13 filled with a volume ofhot water 14 up to a level 15 and with a steam head 16 above the hotwater 14.

The tank 2 is located above the entire pressuriser 13 and above theejector 6 which in turn is located above the level 15 of hot water 14 inthe pressuriser 13 while the condenser 7 is below the level 15.

The nuclear reactor also has a primary circuit of which the hot leg isindicated 17 in the drawings.

The steam head 16 communicates with the upper part 4 of the tank 2through a first connector duct 18.

As a result, the tank 2 and the pressuriser 13 are at the same pressure.

A second connector duct 20 branches from the first duct 18 and isconnected to the inlet section 8 of the ejector 6.

The outlet section 9 of the ejector 6 communicates with the inlet 11 ofthe condenser 7 through a third connector duct 21 while the outlet 12communicates with the hot leg 17 through a first injector duct 22 whichopens thereinto through a spray head 23.

The volume of hot water 14 in the pressuriser communicates through afourth connector duct 19 with the hot leg 17 of the primary circuit.

The condenser 7 being below the level 15 of the water 14 within thepressuriser 13 and the circuit constituted by the succession of thethird connector duct 21, the condenser 7, the first injector duct 22,the hot leg 17 and the fourth connector duct 19 being open, the resultis that the condenser 7 is completely flooded with water from thepressuriser 13 which fills the entire circuit up to a point in the thirdconnector duct 21 at the same height as the level 15.

The lower part 5 of the tank 2 is connected to the narrow section 10 ofthe ejector 6 by a delivery duct 24 which constitutes a syphon circuit.

The delivery duct 24 in fact has an ascending portion 24a, an upperportion 24b and a descending portion 24c which is connected into thenarrow section 10.

The upper portion 24b is located above the tank 2 so that the cold waterin the tank 2 fills the delivery duct 24 up to a point in the ascendingportion 24a located at the same height as the level 3.

With reference in particular to FIG. 1, it may be seen that, when thelevel 15 in the pressuriser 13 falls as a result of an incident whichcauses a loss of water from the primary circuit, the water in the duct21 also falls so as to uncover the tubes 30 in the condenser 7.

When this occurs, the condenser 7 starts to condense the steam fillingit.

This phenomenon causes steam to be drawn from the pressuriser 13 throughthe first connector duct 18, the second connector duct 20, the ejector 6and the third connector duct 21.

The passage of steam through the ejector 6 causes a drop in pressure inthe narrow section 10.

This drop in pressure draws steam from the delivery duct 24 and thusalso causes the water contained therein to rise into the upper portion24b.

When this point is reached, the water in the tank 2 is injected simplyby gravity through the delivery duct 24, the ejector 6, the thirdconnector duct 21, the condenser 7 and the first injector duct 22 and isthen sprayed from the spray head 23 into the hot leg 17 of the primarycircuit.

Consequently the steam present condenses rapidly and causesdepressurising of the nuclear reactor.

It is however possible to imagine numerous variants of thedepressurising system 1 explained above.

With reference to FIG. 2, for example, a second injector duct 25branches from the third connector duct 21 and opens into a pressurevessel 26 which contains the fuel elements of the nuclear reactor, thusconstituting the heat source.

In this variant the first injector duct 22 also opens into the lowerpart of the pressure vessel 26.

In the event of an incident which makes rapid depressurising necessary,the system 1 operates in a manner similar to that described withreference to FIG. 1.

Some of the cold water in the tank 2 however flows through the condenser7 and the first injector duct 22 and some flows through the thirdconnector duct 21 to the second injector duct 25.

The quantity of water which flows through the ducts 22 and 25 may bepredetermined by selection of the sections of the ducts 22 and 25.

In a preferred embodiment, the second injector duct 25 terminates in aspray head 29 within the pressure vessel 26.

With reference to FIG. 3, a third injector duct 27 branches from thethird connector duct 21 and includes a descending portion 27a, a lowerportion 27b and an ascending portion 27c.

The third connector duct 27 opens into the steam head 16 of thepressuriser 13.

The portions 27a, 27b and 27c constitute an hydraulic seal, being fullof water.

This prevents steam from circulating in the third injector duct 27during the triggering of the depressurising system 1.

When the cold water is drawn from the tank 2, some of it flows throughthe condenser 7 and the first injector duct 22 and some through thethird injector duct 27.

As in the depressurising system 1 described in FIG. 2, the quantities ofwater which flow through the ducts 22 and 27 may be predetermined byselection of the sections of the ducts 22 and 27.

In a preferred variant the third injector duct 27 terminates in a sprayhead 28.

Numerous variations and modifications may be made to the depressurisingsystem 1 according to the invention all of which fall within the scopeof protection of the inventive concept, as defined by the followingclaims.

We claim:
 1. A depressurising system (1) for plants operating withpressurised steam and including a steam head (16), by injection of coldwater under gravity from a reservoir (2) located at a position higherthan the pressurised plant and having a delivery duct (24) for deliveryto the pressurised plant, characterised in that it includes an ejector(6) which has an inlet section (8), an outlet section (9) and a narrowsection (10), a condenser (7) which has an inlet (11) and an outlet(12), a first connector duct (18) which puts the reservoir (2) incommunication with the steam head (16), a second connector duct (20)which puts the inlet section (8) of the ejector (6) in communicationwith the steam head (16), a third connector duct (21) which puts theoutlet section (9) in communication with the inlet (11) of the condenser(7), a first injector duct (22) which puts the outlet (12) of thecondenser (7) in communication with the pressurised plant, and in thatthe delivery duct (24) forms a syphon (24a, 24b, 24c), the reservoir (2)communicating with the pressurised plant through the delivery duct (24)and through the narrow section (10) of the ejector (6).
 2. Adepressurising system (1) according to claim 1, characterised in thatthe first injector duct (22) terminates in a spray head (23).
 3. Adepressurising system (1) according to claim 1, characterised in that itincludes a second injector duct (25) which branches from the thirdconnector duct (21) and opens into a pressure vessel (26) of thepressurised plant.
 4. A depressurising system (1) according to claim 3,characterised in that the second injector duct terminates in a sprayhead (29) within the pressure vessel (26).
 5. A depressurising system(1) according to claim 1, characterised in that it includes a thirdinjector duct (27) which branches from the third connector duct (21) andwhich opens into the steam head (16) of the pressurised plant and inthat the third injector duct (27) has a descending portion (27a), alower portion (27b) and an ascending portion (27c).
 6. A depressurisingsystem (1) according to claim 5, characterised in that the thirdinjector duct (27) terminates in a spray head (28) within the steam head(16) .